1. Field of the Invention
The present invention relates to a matrix wiring substrate where a circuit wiring is arranged in a matrix form, and more particularly to a matrix wiring substrate to which electrostatic countermeasure can be established during its manufacture.
2. Description of the Related Art
Recently, the development of flat displays as visual units is particularly drawing the attention. Of such displays, since liquid crystal display devices have many advantages, it has been desired to spur the further development thereof to a leading display system for the future.
Liquid crystal displays employing an active matrix system which include amorphous-silicon thin film transistors (a--Si TFTs) are predicted to take a leading position because of their good display quality. At present, the development thereof is directing toward practical displays from relatively small ones.
The active matrix type liquid crystal display is manufactured by the steps of making a matrix wiring substrate on which pixels electrodes, gate lines and source lines arranged on the pixel electrodes, and thin film transistors (TFTs) are formed in a matrix form over a glass substrate such as an insulator; subjecting the substrate to an assembling step; injecting a liquid crystal material; and connecting drive circuits.
In the steps, static electricity tends to produce between electrodes during the manufacture of the matrix wiring substrate. Since the electrostatic discharge destroys the insulator or semiconductor material for TFT transistor, and the heat generated from the electrostatic discharge damages the circuit wiring, the manufacturing yield of the wiring substrate is largely degraded. Particularly, a--Si TFTs are breakable with respect to static electricity.
Conventionally, as shown in FIG. 10, the guard ring 12 disclosed in Japanese Laid-open Patent Publication No. 58-116573 has been formed in order to prevent the electrostatic damage. After the manufacture of the matrix wiring substrate 10, the outer peripheral portion of the pixel area 14 is cut away together with the glass substrate 24 using a cutting tool such as a diamond cutter. After the guard ring 12 is cut away, the resultant wiring substrate is subjected to post steps including an assembling step and a drive circuit connecting step.
FIGS. 11 and 12 show a structural example where a portion including a gate line 16 and a source line 18 are actually formed on a substrate in a conventional active matrix liquid crystal display device shown in FIG. 10.
The electrostatic countermeasure according to the above mentioned method is effective when the guard ring 12 is formed. However, after the guard ring is cut off in the post steps, since the wiring substrate is exposed to the static electricity, it is susceptible to the static electricity at the post steps (by the time when drive circuit is assembled). Therefore, the possibility that the wiring substrate is damaged remains higher.
Particularly, since the static electricity tends to generate during the LCD assembling process, together with the a--Si TFT transistor forming step, it has been a serious problem that any electrostatic countermeasure is not taken during the assembling step.
In the above mentioned electrostatic countermeasure, although source lines and gate lines are in the same potential, it is uncertain whether the switching element 20 is switched on.
Therefore, when the pixel electrodes 22 is electrostatically charged, a voltage creates between the gate line and the pixel electrode, thus causing a degradation factor of TFT transistors.
Furthermore, when the guard ring 12 is cut away, a static electricity is liable to generate between the glass substrate 24 and a cutting tool, thus causing a damage of the circuit substrate.
While the guard ring 12 is being formed, the circuit wiring cannot be inspected because the source line 18 is short-circuited electrically with the gate line 16. The guard ring 12 has been once cut away on purpose of an inspection, the guard ring 12 cannot be rebuilt so that the circuit wiring becomes open to the static electricity after the inspection. Hence since the possibility that defects occur after the inspection is high, the inspection becomes substantially meaningless. To overcome such a problem, the inspection has been performed after separating off the guard ring 12 and then connecting a drive circuit by way of the post steps including a wiring substrate assembling step. However, according to the above mentioned method, even if a defect in the circuit wiring is found, it is too late to treat it because many steps have been passed. Thus many rejected articles are subjected to disposal treatment, thus leading to large manufacturing loss. Therefore, it has been desirable to inspect the circuit wiring at early stage.